The present invention relates to a thin film semiconductor comprising a dielectic substrate and a single crystal silicon thin film thereon and a process for producing the same.
Such a thin film semiconductor is utilized for constituting various OA (office automation) equipments such as a large sized image scanning and driving device and a displaying and driving device.
Conventionally, it is known to produce a thin film by a plasma CVD (chemical vapor deposition) method, a thermal CVD method, an optical CVD method, an LP-CVD method, an MO-CVD method, a sputtering method or a vacuum evaporation method. In accordance with either method, the produced thin film has a crystal structure which is resembled to that of the substrate. Therefore, when the substrate is made from glass or ceramics, which is most generally used as an electric insulation material, a single crystal thin film can not be formed on the substrate by the above-mentioned method but instead a polycrystalline or amorphous thin film is formed on the substrate.
In order to obtain a single crystal thin film using the glass or ceramic substrate, it has been proposed a ZMR (Zone Melting Recrystallization) method in which the polycrystalline or amorphous silicon thin film is converted to a single crystal thin film by melting and recrystallizing the polycrystalline or amorphous silicon thin film. In accordance with this ZMR method, an advantage is that an invariable crystal direction is predominantly arranged on the crystallized film surface due to the anisotropy of the surface energy of the silicon thin film at the time of melting and recrystallization thereof.
Examples of heating means for melting the silicon film in accordance with the ZMR method are (1) a radio frequency carbon susceptor heating method, (2) an infrared heating method and (3) strip heater method. In accordance with either method, the polycrystalline or amorphous thin film formed on the sutbstrate have to be heated to a temperature more than the melting point of the thin film. Therefore, when a silicon thin film is to be treated, the heating temperature must be more than 1412.degree. C. which is the melting point of silicon.
Such a high temperature process required for recrystallization of the thin film causes thermal unbalance during the process of single crystal growth, which results in that crystal defects are generated in the single crystal thin film and/or that the substrate is deformed by the heat.
For example, micro-cracks are generated on surface of the substrate of crystal glass as one of the crystal defects mentioned above. When a silicon film having such micro-cracks formed therein is used for constituting a transistor device or other electronic devices, characteristic of responsiveness is impaired and reliable function can not be achieved.
The micro-cracks are supposed to be generated by the reason as follows.
The supporting member (substrate) which is made from quartz glass has a coefficient of thermal expansion of about 5.times.10.sup.-7 /.degree.C. Whereas silicon has a coefficient of thermal expansion of about 40.times.10.sup.-7 /.degree.C. Therefore, the amount of expansion and contraction of silicon is larger than that of the quartz substrate in the process of heating to the high temperature and cooling down to the room temperature. However, the thickness of the silicon film is very thin (0.5 .mu.m to 1 .mu.m) when compared with that of the quartz glass substrate (0.5 mm to 1 mm). Therefore, the expansion and contraction of the silicon film in the direction in parallel with the film and substrate surface is restricted by the quartz glass substrate so that strain is generated and remains within the silicon film, which causes to form micro-cracks in the silicon film.
In order to obviate the above-mentioned problems, it is proposed to divide the silicon film to a plurality of separated islands in a size of 25.times.25 .mu.m to 100.times.100 .mu.m so as to the crack generation.
However, when the island of the silicon film is formed in a large size, it becomes difficult to avoid generation of cracks in the island. Therefore, the area of the silicon film is limited to being small so that a silicon film of large size can not be obtained, thus preventing realization of electronic elements of high pattern density since the area for forming the thin film is limited.
The above-mentioned problem is not limited to the case in which the substrate is made from quartz glass but inevitable when the coefficient of thermal expansion of the substrate material is different from that of silicon.
As mentioned before, in accordance with the ZMR method, the silicon thin film is heated and melted by (1) a radio frequency carbon susceptor heating method, (2) an infrared heating method or (3) a strip heater method. Also, in accordance with the ZMR method, it is necessary to slowly cool the molten silicon when solidifying it to obtain a high quality recrystallized thin film. In either of the above-mentioned methods (1) to (3), the substrate is also heated almost to the melting point 1412.degree. C. of silicon so as to slow down the cooling speed of the silicon film.
Therefore, the substrate have to be made from a material which is stable at this temperature. If the substrate is furthermore required to be transparent, the material for constituting the substrate is limited to quartz which has a softening point of 1650.degree. C.
It has been proposed to use a thin film transistor comprising the silicon thin film formed on a predetermined substrate, according as the recent development of electronic devices of large size such as a one-dimensional photo-sensor which is elongated in size for reading or scanning images, a scanning device of an image scanner comprising a two-dimensional photo-sensor having a large scanning area, and a driver device of an image display using a liquid crystal material, an electrochromic material or an electroluminescense material. The silicon thin film of such a transistor is usually made from amorphous silicon or polycrystalline silicon since these silicon materials are relatively easy to form on a a large sized substrate.
It is required to realize the above-mentioned device which functions in a high speed and reliably. Accordingly, it is needed to upgrade the functional guality of the thin film transistor that constitutes a driving circuit of the device. However, the mobility of the amorphous silicon thin film transistor is about 0. 1 to 1.0 cm.sup.2 /v.s. Also, the mobility of the polycrystalline silicon thin film transistor is about 1.0 to 10 cm.sup.2/ v.s. Such mobility of the amorphous silicon transistor or the polycrystalline silicon transistor is far from that of the single-crystalline silicon transistor which mobility is more than 600 cm.sup.2 /v.s.
Accordingly, it is needed to develop a technique to form a single-crystalline silicon thin film on a substrate of large size.